A picture of all the grandkids in bunny ears:
Every Grandparent loves to add to their photo brag book
and this is sure to be keeper.
You can go as far as:

painting on whiskers to all out rabbit suits.
Make sure to have them holding a personalized “I love you Grandma/Grandpa!” sign.
You can even put the picture in a custom hand made photo frame
or purchase an Easter themed one from your local store.